Ascidia subterranea, Kneer, Dominik, Monniot, Francoise, Stach, Thomas & Christianen, Marjolijn J. A., 2013

Kneer, Dominik, Monniot, Francoise, Stach, Thomas & Christianen, Marjolijn J. A., 2013, Ascidia subterranea sp. nov. (Phlebobranchia: Ascidiidae), a new tunicate belonging to the A. sydneiensis Stimpson, 1855 group, found as burrow associate of Axiopsis serratifrons A. Milne-Edwards, 1873 (Decapoda: Axiidae) on Derawan Island, Indonesia, Zootaxa 3616 (5), pp. 485-494 : 487-491

Ascidia subterranea sp. nov.

Etymology. The species name refers to the unusual habitat below the sediment surface, in burrows excavated by the shrimp Axiopsis serratifrons .

Type locality. Derawan Island, off East Kalimantan in the Sulawesi Sea, Indonesia, 2°17’12’’N, 118°14’53’’E.

Syntypes. Six specimens (1–6 as listed below):

Naturkundemuseum, Berlin:

ZMB Tun 4015 (Specimen 1): Animal used for SEM preparations by T.S., remains: complete tunic with most of the coral gravel removed, part of oral siphon, part of branchial basket, larger piece of intestine with small part of branchial basket, small part of intestine, body parts not stained, gonads present. SEM stubs: 1. part of the dorsal lamina from the posterior third with a larger area of the right and a smaller area of the left branchial basket; 2. Oral tentacles and dorsal tubercle; 3. Oral tentacle and anteriormost part of endostyle; 4. Endostyle with a larger area of the left and a smaller area of the right branchial basket.

ZMB Tun 4016 (Specimen 2): Animal in its tunic, in very poor condition, not stained, gonads present.

ZMB Tun 4017: traces of empty tunic belonging to Specimen 2, encrusting coral gravel preserved.

ZMB Tun 4018 (Specimen 4): Animal used for SEM preparations by T.S., remains: tunic, body without dorsal part of the oral siphon and anterior part of the ventral branchial basket, not stained. SEM stubs: 5. oral tentacle, dorsal tubercle, and part of anterior branchial basket; 6. part of the dorsal lamina from the anterior third with a larger area of the left and a smaller area of the right branchial basket.

Muséum National d'Histoire Naturelle, Paris:

MNHN P5 ASC.A 416/1 (Specimen 3): Animal in its tunic, in very poor condition, not stained, beginning of oral siphon and tentacles preserved.

MNHN P5 ASC.A 416/2 (Specimen 5): Tunic removed, posterior part of the body in good condition, not stained, tentacle area missing, internal part of the torn oral siphon stained in search of muscles, dissected by F.M., parts mounted on a slide for light microscopy.

MNHN P5 ASC.A 416/3 (Specimen 6): Tunic removed, body in good condition but opened along the bodyside length, stained, part of the branchial sac mounted on a slide for light microscopy by F.M., oral siphon missing.

MNHN P5 ASC.A 416/4: empty tunic belonging to Specimen 6, encrusting coral gravel preserved.

Description. Six specimens of a phlebobranch ascidian have been extracted from burrows of Axiopsis serratifrons . The ascidian specimens are more or less damaged, as they are soft and almost entirely covered with large coral debris, shells and diverse coarse particles. The tunic was strongly adhering to the wall of the shrimp tunnel (expect, possibly, for the part around the distal end of the oral siphon which was always missing). All individuals have the same elongate shape. The mineral coating is deeply incrusted into the tunic, which is irregular in thickness, cartilaginous or paper-like in few parts. When the tunic is removed, the general body shape is oval about 4.5 cm long prolonged by a tubular oral siphon at least as long as the body itself, but in all specimens torn at its extremity. In few places, where the tunic is free from included gravel or shells the tunic surface wears soft spiny papillae 0.6 to 0.8 mm long ( Fig. 1 B). The oral siphon is a long thin walled tube about 8 mm in diameter. It extends far above the circle of simple oral tentacles, and its rim is always missing, either torn in the process of excavating the burrows during sampling, or perhaps cut off by the shrimp. The atrial siphon, sessile in life, is shortly protruding from the dorsal side of the body when it is removed from the tunic ( Fig. 2 ), at two thirds of the body length. Depending on contraction, the rim of the atrial siphon is smooth or undulating in 6 low lobes. On the body the mantle is extremely thin and transparent except at the muscular belt. The siphon sphincters are weak. The oral one is limited to the tentacle area. Anterior to the tentacle ring, its wall is completely devoid of the longitudinal muscles which are always present at the siphons of all other species in the genus Ascidia , a clearly unique, distinguishing and very surprising character. The atrial sphincter is limited to the short tube of the siphon with thin fibres; it does not spread on the body sides. The left body side distended by the gut is totally devoid of muscular fibres ( Fig. 2 ). On the right side the body musculature forms a belt made of a narrow ribbon of transverse thick and short fibres encircling the body outline ( Fig. 2 ). Its course follows the ventral outline, curving at the end of the body to reach the dorsal midline. It is only interrupted at the aperture of the atrial siphon. Both extremities of this ribbon terminate at the level of the oral tentacles at the narrow oral sphincter. The oral tentacles, of which there are approximately one hundred in three orders of sizes, are simple, long, very close and pressed to each other, and planted on a thick rod (Fig. 3A, B). The peripharyngeal groove lies between two low ridges indented in a deep V. The dorsal tubercle opens in a simple U-shaped slit or a U with horns slightly rolled (Fig. 3A). The neural ganglion is located immediately behind the dorsal tubercle. The dorsal lamina is doubled above the neural ganglion only. It has a smooth rim at its beginning but it is fringed by triangular languets more posteriorly (Fig. 3C); it is sustained on its left side by strong ribs (Fig. 3C); the transverse vessels form similar ribs along the unperforated band of branchial tissue located on the right side of the dorsal lamina. The oesophagus entrance opens clearly before the end of the branchial sac and is circled by the dorsal lamina on its left (Fig. 3E). There is no accessory opening of the branchial sac. The branchial tissue is made of regular square meshes slightly pleated. The longitudinal vessels are thin and low (Fig. 3). At the crossing of the longitudinal and transverse vessels the branchial papillae are particularly short, in buttons, often two-lobed. There are four to eight longitudinal stigmata per mesh. The branchial sac extends far below the level of the gut down to the body end ( Fig. 2 ). The gut occupies a large part of the left body side. The stomach is narrow, extends horizontally, and is not well delimited, prolonged in a vertical intestine, which curves in a narrow loop before entering a very large distended rectum of a brown colour ( Fig. 2 ). This part of the gut is the only pigmented tissue of the entire body. The anus, widely open, has a smooth rim close to the siphon aperture. The top of the gut loop is anterior to the anus level. The gonads, when present, have the ordinary shape of the genus, with the ovary located inside the primary gut loop and the testis lobes scattered on the internal side of the gut. The genital ducts follow the rectum and end at the anus level. In one specimen a large vessel, probably the heart, was obvious in the thicker portion of the left body wall, posterior and parallel to the stomach.

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Remarks. Ascidia subterranea sp. nov. belongs to the Ascidia sydneiensis Stimpson, 1855 species group. All of them are characterized by an elongate and laterally flattened body, a convoluted dorsal tubercle, an inflated rectum and a body musculature essentially consisting of a belt of transverse fibres running along the outline of the right side. Since A. sydneiensis is variable, very common and present world-wide a large synonymy has been accumulated and discussed by numerous authors (Kott 1985, Monniot C. 1987, Monniot C. & F. 1987, Nishikawa 1991); it certainly represents several species.

The distinctive characters of Ascidia subterranea sp. nov. are the long oral siphon in a tube devoid of muscles, a dorsal tubercle which is simple or c-shaped instead of convoluted, a branchial sac extending far below the gut, the anus with a smooth rim and long spiny papillae at the tunic surface scattered between the incrusted material. The ribbon of transverse muscles is more clearly limited and considerably narrower compared to all other specimens of Ascidia sydneiensis described so far. A similar design of the right body side musculature is also present in the species Ascidia munda Sluiter, 1898 but the gut shape is different, the dorsal tubercle is convoluted and the longitudinal muscles are in bands on both long siphons as in A. sydneiensis . The habitat inside the burrow of an axiid shrimp is very uncommon for an ascidian and allied to the peculiarity of morphological characters justify to create a new species.

Figures

FIGURE 1. A Opening of the burrow of Axiopsis serratifrons, with anterior end of the shrimp visible, surrounded by the narrow leafed morph of Halodule uninervis. B Tunic papillae (tpa) of Ascidia subterranea sp. nov. (MNHN P 5 ASC. A 416 / 2). C Schematic representation of the habitat of Ascidia subterranea sp. nov., showing the position of the living tunicate on the burrow roof. Also shown are the pair of shrimps, and associated bivalves, gastropods and polychaetes. Three alternative (all of them speculative) positions for the siphon are depicted.

FIGURE 2. A, B Ascidia subterranea sp. nov. (MNHN P 5 ASC. A 416 / 2), tunic removed seen from the left (A) and right (B) side respectively. C, D Corresponding drawings to the photographs in A and B. an—anus, as—atrial siphon, gd—gonadal ducts, ht (?) — enlarged blood vessel, probably the heart, in—intestine, mb—muscle belt, ov—ovary, re—rectum, sto—stomach